1. Field of the Invention
The systems and methods of this invention relate to electrical stimulation of the heart by means of an implantable device. Specifically, the present invention relates to systems and methods for providing such stimulation without the use of conventional lead/electrode systems. More specifically, the present application provides systems and methods for treatment of heart failure and for terminating heart arrhythmias using external and implantable pacing systems and components.
Electrical stimulation of body tissues is used throughout medicine for treatment of both chronic and acute conditions. A commonly implanted device utilizing electrical stimulation is the cardiac pacemaker. External cardiac pacemakers are also commonly used in acute settings, particularly after cardiac surgery or in emergency situations to provide stimulation of the heart during recovery.
Devices to provide temporary, acute heart stimulation comprise an external pulse generator (EPG), i.e., an external pacemaker, which is connected to leads/wires that have been placed in or on the heart. This invention pertains to devices in which at least one portion providing direct electrical stimulation to the body tissue is either permanently or temporarily implanted in the heart and is controlled by one or more external devices.
EPG devices are connected to electrode/lead wires that have been placed in one or more heart chambers by vascular access or surgical access or placed on the outside (epicardial surface) of the heart during surgical access. Surgical access is accomplished using an open chest procedure or as a minimally invasive procedure. Following heart surgery, it is common for these wires to be placed as a precaution against post surgical arrhythmias and used either to stimulate the heart to maintain a heart rhythm or to terminate abnormal rhythms (e.g., tachyarrhythmias in the atria or ventricles). These wires protrude from the body either near the vascular insertion site or through a chest insertion site left after closure of the surgical procedure. The insertion/exit site of the wire from the patient must be kept sterile. The temporary, acute use of the EPG and wires is referred to as temporary pacing and is used for a variable period; some times up to as long as 10 days. The pacing wires are removed after the patient has stabilized. In patients who require ambulatory pacing for longer periods, an implanted pacemaker and pacemaker leads are used and this approach is generally referred to as permanent pacing. The use of temporary lead wires or pacing leads is associated with significant problems such as complications due to infection, lead failure, and electrode/lead dislodgement.
The requirement for leads in order to accomplish stimulation also limits the number of accessible locations in the body. For example, temporary leads are not placed using arterial access due to the increased risks associated with thrombus, thromboemboli, and embolization in the arterial vascular system. The requirement for leads/wires has also limited the ability to stimulate at multiple sites (multi-site stimulation), which requires even more wires protruding from the body, thus increasing complication risks.
An EPG is a pacemaker that is a battery-powered electronic device typically located and used at the bedside of a patient in need of emergency heart stimulation or as a precaution in treating arrhythmias and heart stimulation needs after surgery, As noted above, the EPG is connected to the heart by an insulated metal lead wire with distal electrodes The wires are connected to terminals at the EPG, which functions in commonly used single and dual chamber pacemaker modes and also in commonly used anti-arrhythmia pacing modalities. Pacemakers were initially developed for and are most commonly used to treat bradycardia, slow heart rates, which may result from a number of conditions, particularly following surgery. More recently, advancements in pacemaker complexity, and associated sensing and pacing algorithms have allowed progress in using pacemakers for the treatment of other conditions, notably heart failure (HF) and fast heart rhythms (tachyarrhythmia/tachycardia). EPGs contain the same or similar operational characteristics as implanted pacemakers including sensing circuits to determine the intrinsic heart activity and adjustments for pacing rate, pace pulse amplitude, pace pulse duration, sensitivity levels for detecting heart beats, pacing modalities, anti-tachyarrhythmia algorithms, and the like.
In a common application such systems can be either single chamber with a lead wire placed in or on either the right atrium or right ventricle, or dual chamber systems with one lead wire placed in or on the right atrial wall and a second lead placed in or on the right ventricular wall. For the treatment of HF, through what is commonly known as cardiac resynchronization therapy, bi-ventricular pacing is utilized, requiring that an additional lead be placed in contact with the left ventricle. Using a vascular access to the left ventricle, the third lead is typically advanced into the right atrium, into the orifice of the coronary sinus, and then maneuvered through the coronary sinus veins to a position on the epicardial aspect of the posteriolateral or lateral wall of the left ventricle, alternatively the third lead is placed epicardially directly in contact on the wall of the left ventricle using a surgical access. Particularly in temporary pacing situations following surgery, the leads/wires are placed on the epicardial aspects of the heart chambers.
A common and difficult problem with using temporary pacing leads is infection. Infections can occur locally at the open skin of the insertion site or can progress down the wire and develop more serious systemic internal infections including pericarditis or endocarditis. Some other common complications in using temporary wires are dislodgement of the lead from the tissue and perforation of the myocardium by the lead/wire.
It has recently been found that biventricular pacing following surgery can help to optimize hemodynamics during the recovery period. This is particularly pertinent in cardiac surgery patients with wall-motion abnormalities and in patients with symptoms of HF. Placement of the third lead to contact the left ventricle remains a significant problem using a vascular access. In cardiac surgery patients, access to the heart allows a direct epicardial attachment of a left ventricular lead. The vascular approach to left ventricle access is not often used for temporary pacing due to the difficulty of the lead placement. The coronary sinus is a complicated venous pathway with multiple branches which bend and narrow with considerable variation as they extend distally onto the epicardium of the left ventricle. Placement of the third lead requires significant skill on the part of the physician. In order to provide adequate steerability and pushability, the design of the left ventricular lead or a lead introduction system/device is much more complicated than regular temporary pacing leads/wires. Often the positioning and placement of the left ventricular lead can take over an hour to perform, exposing the patient to increased fluoroscopy radiation, increased anesthesia exposure, and increased procedure risks.
Left ventricular leads are not placed inside the heart chamber as they are for the right-ventricular leads for several reasons. They would have to be situated retrograde across the aortic valve or trans-septally across the mitral valve which could cause aortic or mitral valvular insufficiency. The patients would be subject to risk of thromboembolic complications from having leads in the arterial circulation. Alternatively, atrial transeptal puncture from the right atrium to insert a pacing lead into the left atrium or left ventricle is also subject to risks of thromboembolic complications and for left ventricular sites, would cause mitral regurgitation. Moreover, all temporary pacemaker leads/wires are associated with an incidence of infection, and the risk of valvular endocarditis is greater in the left heart.
In patients receiving a bi-ventricular pacing system, site selection for placement of the left ventricular lead has been found to be critically important in order to provide improved hemodynamic benefit. Up to 40% of patients receiving bi-ventricular pacing for the treatment of HF do not benefit from the pacing (i.e., hemodynamic measures and HF functional class do not improve or deteriorate). The most important cause for lack of benefit is thought by experts to be due to suboptimal or incorrect left ventricular stimulation site. The ability to precisely select the left ventricular site for stimulation in combination with right ventricular stimulation, would aid in the improvement of hemodynamic benefits following cardiac surgery.
Moreover, left ventricular stimulation currently is restricted to sites on the epicardial (outer) surface of the heart; the coronary sinus courses on the epicardium, and surgically implanted left ventricular leads/wires are screwed into the epicardium. Recent data indicates that endocardial (inside lining) or subendocardial (inside layer) stimulation sites in the left ventricle provide additional hemodynamic benefit.
Importantly, clinical trial data now suggest that pacing of the left ventricle alone may result in hemodynamic benefit equivalent to that of bi-ventricular pacing. Thus, a leadless pacing system has the potential to accomplish the benefit of bi-ventricular pacing without the need for a right ventricular pacing lead or electrodes.
It would also be beneficial to provide more physiological right ventricular pacing for patients to improve hemodynamics following cardiac surgery. In normal physiology, the right ventricle is first stimulated in the upper septal area, and then the impulse travels down specially conducting pathways near the endocardium to the right ventricular apex. However, using a vascular-placed lead/wire, pacing the right ventricle is virtually always accomplished from a lead tip electrode located in the right ventricular apex, such that the subsequent conduction pathway is abnormal and slow. In similar reference to abnormal conduction from the left-sided epicardial stimulation, leads/wires placed surgically on the right ventricle also impact hemodynamic performance. Clinical trials have recently shown that in patients with and without A-V block, pacing from the right ventricular apex can result in increased total mortality and re-hospitalization for heart failure. Thus, it would be advantageous to be able to pace the right ventricle at more physiological locations such as the endocardial aspects of upper septum. The most appropriate physiological location to pace the ventricle in patients with sinus nodal or A-V junction conduction disease is to directly pace the His bundle. In patients with lower conduction disease involving the A-V junction or bundle branches, the most physiological pacing sites have been found to be the left ventricular septum or left ventricular apex. These are locations in proximity to the specialized Purkinje conduction network. These locations are not accessible using current transvenous lead-based pacing systems. It would be advantageous to be able to select the pacing site in order to model more normal conduction and improve hemodynamics following cardiac surgery.
In addition to improving hemodynamics, post cardiac surgery patients often have arrhythmias. Pacing algorithms have been shown to be effective in terminating atrial and ventricular arrhythmias, referred to as antitachycardia pacing (ATP). For ATP, the lead-based system has limitations in selecting the location of the pacing application, particularly in the left side of the heart. Ventricular tachyarrhythmias can be readily terminated using low voltage pacing stimulation if the site of the pacing is near the ventricular tachycardia focus or reentrant circuit. However, this is usually in the left ventricle, and close to the endocardium. As noted above, current temporary pacing devices incorporate antitachycardia pacing modalities, but the pacing site is limited to the right atrial or right ventricular lead or is subject to the same limitations previously described for left-sided lead placements. Further, right-sided locations have been shown to be less effective in electrophysiology laboratory testing, especially for tachyarrhythmias of high rate which are more serious. This ineffective treatment may lead to treating the patient with high-energy cardioversion/defibrillation shocks using an externally applied defibrillator. These high energy shocks are extremely painful and place additional stress on post cardiac surgery patients. Therefore, it would be advantageous to be able to select the pacing site, particularly for ventricular tachyarrhythmias near the endocardium and in the left ventricle. Having the capability to select the location for a left ventricular electrode for terminating episodes of ventricular tachycardia using antitachycardia pacing techniques would be expected to be more effective compared to current devices.
Another limitation of both the requirement of having leads and of having limited access to the left heart is in the emerging area of multisite pacing for termination of atrial fibrillation (AF) and ventricular fibrillation (VF). These arrhythmias typically arise in and are maintained by the left atrium and left ventricle. Studies have demonstrated the presence of excitable gaps within the tissue during atrial fibrillation (animal and human studies) and ventricular fibrillation (animal studies). By placing and stimulating at multiple pacing sites, regional pacing capture can be obtained during these arrhythmias. This means that if stimulation is delivered at the appropriate timing to a sufficient number of sites, in the appropriate locations, termination of atrial and ventricular fibrillation is possible. The advantage of terminating fibrillation with selected site left ventricular pacing would be the avoidance of painful high energy shocks. In this application the capability for left-sided stimulation and multi-sites of stimulation would be advantageous.
In addition to the termination of tachyarrhythmias, pacemaker algorithms have been used to prevent tachyarrhythmias. In post cardiac surgery patients AF is a typical complication. In patients receiving permanent pacemakers, the dual chamber (DDD) mode has been shown to result in fewer episodes of AF compared to single chamber (VVI) mode in several large clinical trials. DDD pacing that incorporates simultaneous multisite stimulation of both the high right atrium and CS ostium has also been compared to standard single atrial site DDD pacing for the suppression of AF, showing a modest reduction of AF episodes. Atrial stimulation at a site or multiple sites may be advantageous for the prevention of atrial fibrillation by shortening total atrial activation time. Right atrial sites in Koch's triangle and Bachman's bundle may reduce atrial activation time by stimulating near or within atrial conduction tracts or within other tracts that are part of the normal conduction pathway. In an experimental canine model (Becker), either 4 pacing sites (2 in RA and 2 in LA) or one in the interatrial septum were required for suppression of AF. While these results are very promising, they present a technical obstacle for current pacemaker systems. The use of multisite pacing incorporating pacing sites in the left atrium for the suppression of AF has not been evaluated in humans because of all the issues of using multiple leads and in using leads within the left heart.
It follows that if AF may be suppressed with multisite atrial pacing (especially in the left atrium), then VF may also be suppressed with multisite ventricular pacing (especially in the left ventricle). However, the difficulties associated with the implantation of multiple leads in the left ventricle has rendered this form of prevention almost impossible.
For these reasons, it would be desirable to accomplish stimulation without lead wires. In this application we describe methods and apparatus, using acoustic energy in combination with an implantable leadless stimulator and an external control system that overcome limitations in pacing site selection. In co-pending applications we further describe improved stimulating devices. Methods and systems to evaluate and optimize positioning for implantation of this invention are described herein.
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